After years of research and development, radio frequency light sources are just now becoming a mainstream lighting option.
Lighting isn't exactly the first thing you think of when you hear “radio frequency.” More comfortable in the lexicon of HAM radio operators, the term is just now making headway in the business of illumination, thanks to designs that employ RF technology and offer appreciably longer life and lower maintenance. Even at these early stages of development, RF lamps are finding a foothold in applications traditionally dominated by fluorescent and high-intensity discharge (HID) lamps, giving lighting designers a larger pallet of products from which to choose on their next project.
Beyond broadening the range of lighting choices, though, RF light sources offer another benefit: increased functionality. Given the shortcomings of conventional discharge lamps, it's not difficult to see why interest has increased in such a technology that succeeds where fluorescent and HID sources have failed.
A typical hot cathode fluorescent lamp has a cathode — also called an electrode — located at each end of the glass tube. The electrode consists of a tungsten filament, similar to that of an incandescent lamp. Unlike its purpose in an incandescent lamp, here the filament's purpose is to emit electrons, not to directly produce illumination. And in this case, the cathode filament has an emissive material coating to accelerate the electrons' release.
The resistance of the path, or gap, between the two electrodes is broken down when sufficient voltage is created between the electrodes. And the resulting continuous mercury arc, or ultraviolet energy (electromagnetic radiation), created along the length of the lamp activates the phosphor coating on the lamp's interior surface. These phosphors receive the invisible ultraviolet energy and change it into visible energy.
Strictly speaking, the coil, or electrode, at each end of a fluorescent lamp is technically a cathode only while it gives off electrons, which only lasts for one half of the AC cycle. For the other half, it receives electrons and thus functions as an anode. However, the industry uses the term cathode for simplicity.
During lamp operation, the mercury arc will contact the cathode continuously at the same spot. Over time, the arc will gradually wear away the coating material at this spot; when the coating has been completely worn away at that spot, the arc will move to another area of the cathode surface. Simply starting the lamp also damages the coating by wearing away the emissive material. When a cathode filament loses all of its coating, the lamp has reached the end of its life. So, generally, the loss of the electrode coating determines lamp failure.
Using an electronic ballast can reduce cathode coating loss, and in many instances extend average fluorescent lamp life. But, still, in the case of frequent lamp ON/OFF switching, such as with occupancy sensor control, the cathodes will wear out faster than if the lamp switches on only a few times during a day. A key advantage of the electronic ballast is the high-frequency operation of the lamp, which increases the efficiency of the phosphor conversion to visible light.
Nevertheless, some of the latest premium electronic ballasts can soft start a fluorescent lamp, reducing the damage to the coating. Under optimum conditions, a matched lamp/ballast system can offer a 30,000-hr average rated life for T8 linear lamps.
Considering the difficulties created by fluorescent lighting, it should come as no surprise that researchers have been trying to develop a practical electrode-less light source for years. The main obstacle to the development of a commercial RF lamp was the lack of efficient and economical electronic components to drive the lamp at frequencies as high as 60 Hz, the level necessary to produce visible light more efficiently. The past 15 years have seen the development of power switching electronics, which is revolutionizing many facets of the electrical industry, and a better understanding of RF plasma characteristics, making it possible to drive lamps at those frequencies.
Within the past 10 years, the following RF lamp designs have achieved varying degrees of commercial use:
Microwave-powered sulfur lamps generate a full-color spectrum directly by exciting sulfur molecules within a microwave resonant cavity and without the use of a phosphor coating. A 1.5kW magnetron, similar to those used in microwave ovens, generates power at 2.45 GHz and delivers energy to a quartz glass bulb through a short wave guide (Fig. 1). Filled with argon and a small dose of sulfur, the bulb (about 3 cm in diameter) is rotated for discharge stability within the resonant cavity. Providing about 135,000 lumens, or 95 lm/W, and a life rating of 15,000 hr, the compact sulfur lamp also features low-infrared and ultraviolet emission and good color stability. Installations to date have been demonstration projects only.
Spherical external coil induction lamps employ a 4.5-cm diameter fluorescent bulb driven inductively at 13.56 MHz from an RF power supply housed in a base unit (Fig. 2). An induction coil wrapped around the lamp energizes the neon gas and a small quantity of mercury contained within the lamp. In turn, a screen cage surrounds the lamp to reduce EMI emissions to an acceptable level. The system operates at 27W, and the efficacy is 37 lm/W. Typical applications would be difficult-to-reach locations, such as a bridge or a room with a high ceiling.
Self-ballasted re-entrant cavity lamps take the place of incandescent reflector lamps, and are slightly smaller than a 75W R30 flood lamp. Housed in the lamp base, an electronic ballast operates at 2.65 MHz and delivers an initial light output of 1,100 lumens, or about 48 lm/W (Fig. 3). Using this technology, one manufacturer has developed a reflector flood lamp that boasts a 15,000-hr average rated life, a CRI of 82, and is available with phosphors providing either a 2,700°K or a 3,000°K color temperature.
Low-frequency extended-coil induction lamps offer advantages over a single-coupling antenna. Available in two wattages, the lamps consist of a 5.4-cm diameter Pyrex glass tube constructed with a rectangular or stretched-donut shape (Fig. 4). Two ferrite coils located on the shorter sides of the rectangular lamp provide the energy coupling. The power transfer efficiency of this unit is as high as 98%. For example, the 150W lamp offers 80 lm/W efficacy.
The lamp's operating frequency of 250 kHz minimizes the problems associated with EMI, and the ballast design is much simpler than an RF system working at 2.65 MHz. This induction lamp is particularly useful for applications on bridges, tunnels, high-mounted street luminaires, and similar difficult-to-reach locations.
Re-entrant cavity induction lamps operate at 2.65 MHz and are available in three wattages — 55W, 85W, and 165W. All three are shaped like standard incandescent “A” lamps (Fig. 5). The 85W model is 11 cm in diameter and 18 cm long.
An induction coil is wound on a ferrite core with an internal copper heat conductor connected to the lamp base and located in the center of the lamp. A heat conductor removes heat from the re-entrant cavity and the induction coil. A 40-cm coaxial cable delivers power from the electronic ballast to the base of the lamp. By separating the two components, the ballast operates cooler, which extends its life. With its vibration resistance, an efficacy of over 75 lm/W and a 100,000-hr average rated life, this induction lamp is particularly useful for applications where regular maintenance of lighting equipment is difficult. A typical application of 165W induction lamps would be pole-mounted luminaires for dusk-to-dawn illumination on a campus.
RF sources are by no means taking over the lighting industry — for that matter, they aren't even a major player yet. However, after years of research and development, lighting engineers are finally able to offer a lighting source alternative that corrects for many of the failures of conventional sources. And who would have guessed it would come about thanks to radio technology?
Sidebar: Unusual Application of Induction Lighting
A unique facade lighting installation that uses QL lamps provides dramatic visual identification for 5 Times Square, a 50-story office tower in New York City. A 5-ft illuminated stripe extends from ground level to the top of the building's north face, which looks out over Times Square. This lighted stripe, running upward at a slight diagonal from left to right, consists of 80 square metal enclosures installed slightly offset. Each enclosure contains an 85W-rated induction lamp. A ⅜-in.-thick translucent acrylic plastic panel, or lens, mounted within a metal frame, protects the lamp enclosure. For lighting maintenance, special tools are used to remove the metal frame holding the fixture lens in place.
The installing electrical contractor was E-J Electric Installation Co., Long Island City, N.Y.